Figure 1. Variable Gain Amplifier, Simplified Schematic

When the 20 K potentiometer is set to zero, the amplifier has a gain of +1/2; when set to

20 K, the gain is -l/2; and when set to 10 K, the gain is zero. FETs Ql to QlO switch in different 20 K potentiometers for the five bands. The outputs for the two segments at U2 are summed together in Ul. A waveform similar to Figure 2 is present at the output of U2B if a full band is swept (for example, 5.8 to 12.5 GHz), and the oscilloscope horizontal input is connected to the sweep output of the analyzer. Similarly, the output of U2A will appear as shown in Figure 3.

Slope Output @I

Both waveforms (Figure 2 and Figure 3) are summed together at Ul to form a current source with Q16 to drive the slope attenuator in the A6A3 Last Converter. The average output current is set by slope gain adjustment R84.

The slope attenuator in the A6A3 Last Converter varies the gain of the 21.4 MHz IF with the collector current of Ql6 providing gain correction that varies with frequency. Within each band, there is independent control over two halves of the band, and IF gain that increases or decreases with increasing frequency.

When Band F (external mixer) is selected, QlS and Q21 provide additional gain to make up for the conversion loss of the external mixer.

A6All 3